Large scale solar power plants utilizing concentrating solar power (“CSP”) technology have been in production for over twenty years. The Solar Electric Generating Systems (“SEGS”) facilities in the Mojave Desert of California are a well-known example of solar power plants using such CSP technology. Concentrating solar power utilizes solar collectors comprising large mirrors or lenses which concentrate solar energy upon an unpressurized pipe or tube that contains a heat transfer fluid. Typically, a synthetic oil having a high boiling point is used as the heat transfer fluid. For example, the SEGS facilities utilize Therminol® from Solutia, Inc. as the heat transfer fluid.
As the heat transfer fluid flows through the unpressurized pipe inside the solar collectors, it is heated by the incident sunlight. One or more pumps are situated along the pipe to pump the fluid through the solar collectors and towards a boiler or heat exchanger. There, the transfer fluid is used to heat water in the boiler to produce steam. The steam is then used for powering a conventional steam turbine to produce electricity. After the heat transfer fluid releases its thermal energy in the boiler/heat exchanger, the heat transfer fluid is pumped back to the solar collectors to be heated once again.
One disadvantage of the use of a synthetic heat transfer fluid is that the fluid has a relatively low energy density. For example, Therminol® has an energy density of approximately 2100 J/kg° C. whereas ordinary water has an energy density of approximately 4200 J/kg° C. This relatively low energy density for Therminol® means that it can carry relatively less thermal energy from the solar collectors to the heat exchanger than water.
Another disadvantage of synthetic heat transfer fluids is that they are often flammable. As a result, care must be taken in handling the fluids and they must be prevented from overheating.
For these and other reasons, a number of solar power systems have recently been developed to produce steam directly from water rather than using a synthetic heat transfer fluid. Such systems—dubbed Direct Solar Steam generation (“DISS”) or Direct Steam Generation (“DSG”)—distribute water through the unpressurized pipes in the solar collectors rather than distributing a synthetic heat transfer fluid. Because water has a much lower boiling point than a synthetic heat transfer fluid, the water will eventually turn to steam after being heated a sufficient amount. Thereafter, the steam is directed to a steam turbine for generating electricity.
Such DSG systems have their own drawbacks, however. First, the presence of steam in the pipes of the solar collectors reduces the efficiency of the collectors because steam has a significantly lower heat capacity than water. Thus, the steam can carry less thermal energy towards the turbine than can pressurized water. Second, the use of a two-phase (water/steam) flow within the pipes of the solar collectors creates a condition known as the Ledinegg Instability. This phenomenon results in a boiling front as the water moves through the pipes and flashes over to steam. To compensate for this instability, an undesirable pressure drop must be introduced into the system. Finally, DSG systems are more sensitive to variations in solar flux density and changes in atmospheric conditions because the systems will not function properly unless the water in the solar collectors is sufficiently heated to flash over to steam. Taken together, these drawbacks necessitate the use of larger, more expensive solar collectors to produce a given amount of electricity. Therefore, such DSG systems may have little or no cost savings in comparison to traditional CSP systems containing synthetic heat transfer fluid.